A polyester film has excellent mechanical properties, heat resistance and chemical resistance. Therefore, it has been used for magnetic tapes, strongly magnetic thin tapes, photographic films, packing films, films for electronic parts, electrical insulating films, films for metal laminate, and films for protecting various members. In recent years, the polyester film has been used for display surfaces of image display devices. For the display surfaces of image display devices such as liquid crystal display devices, glass substrates have been used. However, the use of the polyester film has been investigated in view of the recent trends of reducing the size, light-weighting, and enlarging the screens, of the image displaying devices.